1. Field of the Invention
The present invention relates to a self-excited switching power supply circuit.
2. Description of the Related Art
Conventionally, self-excited switching control has been widely employed as a switching control method for a switching power supply (e.g., Japanese Laid-Open Patent Publication No. 2003-61343). Typically, a self-excited switching power supply is provided with a startup circuit for turning ON a switching element at power-ON so as to bring the switching element to a continuous ON/OFF switching state.
FIG. 29 shows an example of a self-excited switching power supply (push-pull type). The configuration of the switching power supply will be described briefly. The switching power supply includes a power supply-receiving circuit 41 for receiving a commercial power supply, and input-side rectifier circuit 42 is connected to the power supply-receiving circuit 41 for converting an alternating-current voltage to a direct-current voltage. On the downstream side of the input-side rectifier circuit 42, an electrolytic capacitor Ce is connected in parallel to the input-side rectifier circuit 42, with the cathode side of the electrolytic capacitor Ce being connected to the middle point of a main winding Ta′ wound around on the primary winding side of a high-frequency transformer T′ via an inductor Lc.
One end of the main winding Ta′ is connected to a first transistor Q11 of an NPN type, for example, and the other end thereof is connected to the second transistor Q12. The base terminal of a first transistor Q11 is connected to a startup resistor Re, which is connected to the cathode side of the electrolytic capacitor Ce, and is connected to one end of a first auxiliary winding Tb′ via a resistor R21 and a capacitor C11, which are connected in series with each other. The base terminal of a second transistor Q12 is connected to the other end of the first auxiliary winding Tb′ via a resistor R22 and a capacitor C12, which are connected in series with each other. A resistor R23 is provided between the base terminal of the first transistor Q11 and a 0 V line, and a resistor R24 is provided between the base terminal of the second transistor Q12 and the 0 V line. Flywheel diodes D11 and D12 are provided between the collector and the emitter of the first transistor Q11 and the second transistor Q12, respectively.
An output-side rectifier circuit 43 is connected to the secondary winding of the high-frequency transformer T′ for converting an alternating-current voltage induced by the secondary winding to a direct-current voltage, and a smoothing circuit 44 including the smoothing choke coil Ld and a smoothing capacitor Cf is connected to the output-side rectifier circuit 43.
The switching power supply operates as follows. While receiving no commercial power supply, there is no voltage at the bases of the first and second transistors Q11 and Q12, and therefore the first and second transistors Q11 and Q12 are OFF and not in a switching state.
When a commercial power supply is applied, the commercial power supply is rectified by the input-side rectifier circuit 42, thus generating a direct-current voltage V+. The direct-current voltage V+ produces a current flow through the startup resistor Re and the resistor R23. Then, the direct-current voltage V+ increases, and when it exceeds the base-emitter threshold voltage of the first transistor Q11, a current starts to flow through the base of the first transistor Q11.
When a current flows through the base of the first transistor Q11, the first transistor Q11 starts transitioning to the ON state, and a current i flows through the main winding Ta′ of the high-frequency transformer T′ (see FIG. 29). When a current flows through the main winding Ta′ of the high-frequency transformer T′, an induced voltage is generated across the first auxiliary winding Tb′.
The induced voltage further increases the base potential of the first transistor Q11, whereby the first transistor Q11 rapidly transitions to the ON state. Since the electromotive force is of such a polarity that the base potential of the second transistor Q12 is decreased, the second transistor Q12 is reverse-biased and the second transistor Q12 remains OFF.
While the first transistor Q11 is ON, the exciting current through the main winding Ta′ of the high-frequency transformer T′ increases over time. However, the effective magnetic permeability decreases as the magnetic saturation region of the core of the high-frequency transformer T′ is approached. Since this decreases the amount of magnetic flux change, the voltage induced by the first auxiliary winding Tb′ decreases, thereby lowering the base potential of the first transistor Q11.
As the base current of the first transistor Q11 decreases so that the ON state of the first transistor Q11 can no longer be maintained, the exciting current through the main winding Ta′ switches from increasing to decreasing, thereby inverting the polarity of the electromotive force of the first auxiliary winding Tb′. Thus, the base potential of the first transistor Q11 decreases, and the first transistor Q11 rapidly transitions to the OFF state. The base potential of the second transistor Q12 increases, and the second transistor Q12 rapidly transitions to the ON state. Thereafter, the polarity of the electromotive force of the first auxiliary winding Tb′ is inverted repeatedly so as to alternately turn ON/OFF the first and second transistors Q11 and Q12.
In order for the switching power supply to transition from a state where the first and second transistors Q11 and Q12 are not in a switching state to another state where they are in a stable switching state, it is necessary to select an appropriate value for the resistance of the startup resistor Re.
Specifically, in the first transistor Q11, it is necessary that the forward bias voltage from the startup resistor Re is canceled by the inverted electromotive force of the first auxiliary winding Tb′, thereby reliably turning OFF the first transistor Q11. However, if the value of the startup resistor Re is too small, the electromotive force occurring in the first auxiliary winding Tb′ may not be able to produce a sufficient voltage for inverting the ON/OFF state of the first and second transistors Q11 and Q12. Then, the first transistor Q11 may not be able to be turned OFF, thus maintaining the ON state of the first transistor Q11.
If the first transistor Q11 remains ON, the current increases to such a degree that the collector current of the first transistor Q11 is restricted by the series resistance of the internal circuit. Then, the first transistor Q11 may break down due to an overcurrent.
If the value of the startup resistor Re is too large, the system cannot even be started in some cases. Specifically, in order to turn ON the first transistor Q11, it is necessary to produce a current flow through the base of the first transistor Q11 such that the direct-current voltage V+ exceeds the base-emitter threshold voltage of the first transistor Q11. However, if the value of the startup resistor Re is too large, it becomes hard for a current to flow through the base of the first transistor Q11.
Therefore, the value of the startup resistor Re needs to be such that it produces a current flow sufficient to properly turn ON the first transistor Q11 at startup while it decreases the base current of the first transistor Q11 to finally turn OFF the first transistor Q11 as the induced voltage across the first auxiliary winding Tb′ decreases.
As described above, with a switching power supply receiving a commercial power supply (e.g., AC 100 V), the direct-current voltage V+ is dependent on the voltage of the commercial power supply. Therefore, the direct-current voltage V+ will vary and fluctuate significantly. Since the startup resistor Re receives the direct-current voltage V+ directly supplied thereto, a fluctuation of the direct-current voltage V+ also varies the current flow through the startup resistor Re, in which case it will be even more difficult to reliably turn ON/OFF the first transistor Q11.
Moreover, since the startup resistor Re is statically present in the circuit even while the first and second transistors Q11 and Q12 are repeatedly turned ON/OFF, not a small amount of power is consumed due to the resistance of the startup resistor Re.
It is therefore an object of the present invention to provide a self-excited switching power supply in which switching elements can be reliably started and brought to an ON/OFF switching state.